In 3GPP-LTE (3rd Generation Partnership Project Radio Access Long Term Evolution, hereinafter referred to as “LTE”), OFDMA (Orthogonal Frequency Division Multiple Access) is used as a downlink communication method, and SC-FDMA (Single Carrier Frequency Division Multiple Access) is used as an uplink communication method (see Non-Patent Literature 1, 2, and 3, for example).
In LTE, a radio communication base station apparatus (hereinafter abbreviated to “base station”) performs communication by allocating a Resource Block (RB) within a system band to a radio communication terminal apparatus (hereinafter abbreviated to “terminal”) in time units called subframes. Also, a base station transmits control information for notifying downlink data and uplink data resource allocation results to a terminal. This control information is transmitted to a terminal using a downlink control channel such as a PDCCH (Physical Downlink Control Channel), for example. Here, each PDCCH occupies a resource comprising one or a consecutive plurality of CCEs (Control Channel Elements). In LTE, a number of CCEs occupied by a PDCCH (linked number of CCEs: CCE aggregation level) is selected as one of 1, 2, 4, or 8, according to the number of information bits of control information or the channel state of a terminal. In LTE, a frequency band having a maximum width of 20 MHz is supported as a system bandwidth.
Also, a base station transmits a plurality of PDCCHs simultaneously in order to allocate a plurality of terminals to one subframe. At this time, the base station transmits a CRC bit masked (or scrambled) by a transmission-destination terminal ID, included in a PDCCH, in order to identify a transmission-destination terminal of each PDCCH. Then a terminal performs blind decoding of a PDCCH by demasking (or descrambling) a CRC bit with that terminal's terminal ID in a plurality of PDCCHs for which there is a possibility of that terminal being addressed.
Also, a method has been investigated that limits CCEs subject to blind decoding for each terminal in order to decrease the number of blind decoding operations by a terminal. With this method, a CCE area (hereinafter referred to as “search space”) that is subject to blind decoding is limited for each terminal. In LTE, a search space is set randomly for each terminal, and a number of CCEs included within a search space is defined for each PDCCH CCE aggregation level. For example, for CCE aggregation levels 1, 2, 4, and 8, respectively, the number of CCEs included within a search space—that is, the number of CCEs subject to blind decoding—is limited to six candidates (6 (=1×6) CCEs), six candidates (12 (=2×6) CCEs), two candidates (8 (=4×2) CCEs), and two candidates (16 (=8×2) CCEs), respectively. By this means, each terminal need only perform blind decoding on CCEs within a search space allocated to that terminal, enabling the number of blind decoding operations to be decreased. Here, a search space of each terminal is set using a terminal ID of each terminal, and a hash function, which is a function that performs randomization.
Also, standardization has begun on 3GPP LTE-Advanced (hereinafter referred to as “LTE-A”), which implements still higher communication speeds than LTE. In LTE-A, a maximum downlink transmission speed of 1 Gbps or above and a maximum uplink transmission speed of 500 Mbps or above are implemented, offering the prospect of base stations and terminals (hereinafter referred to as “LTE+ terminals”) capable of communication at a wideband frequency of 40 MHz or above being introduced. Also, an LTE-A system is required to accommodate not only LTE+ terminals but also terminals compatible with an LTE system (hereinafter referred to as “LTE terminals”).
In LTE-A, a band aggregation method has been proposed whereby a plurality of frequency bands are aggregated in performing communication in order to implement wideband communication of 40 MHz or above (see Non-Patent Literature 1, for example). For example, a frequency band having a width of 20 MHz is assumed as a basic communication band unit (hereinafter referred to as a “component band”). Therefore, in LTE-A, for example, a 40 MHz system bandwidth is implemented by aggregating two component bands. Also, both an LTE terminal and an LTE+ terminal can be accommodated in one component band.
Also, in LTE-A, the following two methods have been investigated as notifying methods whereby resource allocation information of each component band is notified to a terminal from a base station (see Non-Patent Literature 4, for example). In the first notifying method, a base station notifies resource allocation information of a plurality of component bands to a terminal using a downlink component band of each component band. Then a terminal that performs wideband transmission (a terminal that uses a plurality of component bands) obtains resource allocation information of a plurality of component bands by receiving a PDCCH placed in a downlink component band of each component band.
On the other hand, in the second notifying method, a base station notifies resource allocation information of a plurality of component bands to a terminal using only one downlink component band (20 MHz component band). Then a terminal that performs wideband transmission (a terminal that uses a plurality of component bands) obtains resource allocation information of a plurality of component bands by receiving only a PDCCH placed in one downlink component band. In this case, a terminal need only receive a PDCCH placed in one downlink component band, enabling the number of blind decoding operations to be decreased.